1. Field of the Invention
The present invention relates to an image recording device in which an image is recorded on a printing plate precursor wound around a peripheral surface of a rotating drum, by moving a recording head in the axial direction of the rotating drum while the rotating drum is rotated at a predetermined speed, with the recording head being disposed facing the peripheral surface of the rotating drum and including an optical system that irradiates the printing plate precursor with a light beam modulated on the basis of image data.
2. Description of the Related Art
Devices for exposing printing plate precursors have been developed in which, using sheet-like recording material, and particularly a printing plate precursor comprising a support having disposed thereon a photosensitive layer, an image is recorded with a laser beam or the like directly on an emulsion surface that is a recording layer of the printing plate precursor, by winding the printing plate precursor around a rotating drum and moving a recording head in the axial direction of the rotating drum (sub-scanning) while the rotating drum is rotated at a high speed (main scanning). With such technology, it has become possible to quickly record an image on a printing plate precursor.
The laser beam is controlled by an optical system so that diffused light emitted from an emission point in the recording head converges and is focused at a predetermined focal point position. The focal point position lies on an image recording surface of the printing plate precursor wound around the peripheral surface of the rotating drum. Theoretically, the focal length is such that the focal point stays on the image recording surface of the printing plate precursor as long as the rotating drum rotates without displacing its own axis and the recording head moves along the axis of the rotating drum.
In actuality, however, while the recording head moves along a ball screw shaft, the relative position of the recording head with respect to the rotating drum may vary due to flexion of the shaft. As a result, the laser beam may fall outside the ideal range of the focal depth, whereby image quality is compromised.
In order to solve this problem, it has been proposed to employ an auto-focus device for monitoring the relative position of the recording head with respect to the rotating drum and adjusting the focal length.
An auto-focus device comprises, in the case of triangulation, a laser diode (LD) light source, which is relatively powerful and has a small beam diameter, and a photosensitive diode (PSD) that electrically detects the displacement of the focal point of light, which is emitted from the LD light source and reflected on the printing plate precursor, the displacement of the focal point of light being caused due to the displacement of the printing plate precursor in the thickness direction thereof, and the auto-focus device is complicated and expensive.
In view of the aforementioned circumstances, an object of the present invention is to provide an image recording device in which variation in focal length due to fluctuation in the relative position of a recording head with respect to a rotating drum can be compensated without using an auto-focus device or the like to detect in real time the focal point of a light beam.
A first aspect of the invention is a device for recording an image on a sheet-like recording material in accordance with image data, the device comprising: a rotatably supported drum including a peripheral surface on which the sheet-like recording material is wound; a recording head including an optical unit that receives the image data and irradiates the sheet-like recording material with a light beam modulated on the basis of the image data to record an image on the sheet-like recording material, the recording head disposed facing the peripheral surface of the drum and movable in the axial direction of the drum; a traveling amount detector for detecting a traveling amount of the recording head in the axial direction thereof from a predetermined position; a memory for storing data for compensating for displacement of the optical unit in the direction of the optical axis in correspondence with the traveling amount of the recording head; and a focal point adjusting mechanism for adjusting the focal point of the light beam by moving at least a part of the optical unit included in the recording head in the direction of the optical axis, wherein the focal point adjusting mechanism corrects the focal point based on the traveling amount of the recording head detected by the traveling amount detector, and on the data for compensating for displacement of the optical unit in the direction of the optical axis stored in the memory, in correspondence with the traveling amount of the recording head. The traveling amount detector may include a rotational position detector which detects the rotational position of the drum.
The traveling amount of the recording head in the axial direction of the rotating drum may be computed based on a signal outputted for each predetermined number of drum rotations by the traveling amount detector.
Further, data for compensating the displacement of the optical unit in the direction of the optical axis stored in the memory in accordance with the traveling amount of the recording head may be measured and stored before starting the image recording.
According to the first aspect of the present invention, displacement of the focal point of the light beam is measured in advance by moving the recording head parallel to the axis of the rotating drum while rotating the rotating drum after the device of the present embodiment is assembled. Then, based on the displacement of the focal point of the light beam, the data for compensating the displacement of the optical unit in the direction of the optical axis is prepared and stored in the memory.
The focal point adjusting mechanism is controlled so as to correct the focal point based on the data for compensating the displacement in the direction of the optical axis.
In the first aspect of the present invention, because the displacement of the focal length is measured in advance after the device is assembled, correction of the focal point can, to some extent, be conducted. Although the accuracy of the correction in the first aspect of the present invention is lower than that of real-time correction, (in which the displacement of the focal point is measured and corrected for each scan-exposing) displacements in focal points can be sufficiently compensated using a simple structure while maintaining image quality.
The first aspect of the present invention may further include a non-image recording area recognizing means which recognizes a non-image recording area on the rotating drum based on the rotational position of the rotating drum detected by the rotational position detector. In this case, the focal point is corrected when the recording head faces the non-image recording area for each predetermined number of rotations.
When an image is recorded, the non-image recording area on the rotating drum is recognized based on the rotational position of the drum, which is detected by the rotational position detector. Because the sheet-like recording material is held by, for example, chucks at both leading and trailing ends thereof, the peripheral surface of the rotating drum has non-image recording areas which include at least the portions where the chucks are provided. The focal point adjusting mechanism is controlled so as to adjust the focal point based on the data for compensating the displacement in the direction of the optical axis on the non-image recording area for each predetermined number of rotation of the rotating drum.
Further, the focal point adjusting mechanism may adjust the focal point by changing the relative position of the entire optical unit in relation to the rotating drum.
In this case, because the entire optical unit is moved to change the position relative to the rotating drum, it becomes unnecessary to consider variation or deformation in the focal spot diameter. Thus the image quality can be stabilized as compared to a case in which the focal length is adjusted by moving a part of the lenses in the optical unit.
The image recording device of the first aspect of the present invention may further include a temperature detector provided on or near the recording head. In this case, a correction coefficient based on temperature readings detected by the temperature detector supplements the data for compensating the displacement of the optical unit in the direction of the optical axis.
During operation, the temperature may change in the vicinity of the recording head. When the detected temperature differs from that preset in the data for compensating the displacement of the optical unit in the direction of the optical axis, the difference may be used as a correction coefficient and calculated in the data to make the correction even more accurate.